The present invention relates to a gas mixture formed solely from helium and nitrogen and to its use in a laser welding process operating at a maximum power of 8 kW.
Laser beam welding is a very high-performance joining process as it makes it possible to obtain, at high speeds, very great penetration depths compared with other more conventional processes, such as plasma welding, MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding.
This is explained by the high power densities involved when focusing the laser beam by one or more mirrors or lenses in the joint plane of the workpieces to be welded, for example power densities that may exceed 106 W/cm2.
These high power densities cause considerable vaporization at the surface of the workpieces which, expanding to the outside, induces progressive cratering of the weld pool and results in the formation of a narrow and deep vapour capillary called a keyhole in the thickness of the plates, that is to say in the joint plane.
This capillary allows the energy of the laser beam to be directly deposited depthwise in the plate, as opposed to the more conventional welding processes in which the energy deposition is localized on the surface.
In this regard, the following documents may be cited: DE-A-2 713 904, DE-A-4 034 745, JP-A-01048692, JP-A-56122690, WO 97/34730, JP-A-01005692, DE-A-4 123 716, JP-A-02030389, US-A-4 871 897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693, JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553, WO-A-98/14302, DE-A-3 619 513 and DE-A-3 934 920.
This capillary is formed from a metal vapour/metal vapour plasma mixture, the particular feature of which is that it absorbs the laser beam and therefore traps the energy within the actual capillary.
One of the problems with laser welding is the formation of a shielding gas plasma.
This is because the metal vapour plasma, by seeding the shielding gas with free electrons, may bring about the appearance of a shielding gas plasma which is prejudicial to the welding operation.
The incident laser beam may therefore be greatly disturbed by the shielding gas plasma.
The interaction of the shielding gas plasma with the laser beam may take various forms but it usually results in an effect whereby the incident laser beam is absorbed and/or diffracted and this may lead to a substantial reduction in the effective laser power density at the surface of the target, resulting in a reduction in the penetration depth, or even in a loss of coupling between the beam and the material and therefore a momentary interruption in the welding process.
The power density threshold at which the plasma appears depends on the ionization potential of the shielding gas used and is inversely proportional to the square of the wavelength of the laser beam.
Thus, it is very difficult to weld under pure argon with a CO2-type laser, whereas this operation may be carried out with very much less of a problem with a YAG-type laser.
In general, in CO2 laser welding, helium is used as shielding gas, this being a gas with a high ionization potential and making it possible to prevent the appearance of the shielding gas plasma, and to do so up to a laser power of at least 45 kW.
However, helium has the drawback of being an expensive gas and many laser users prefer to use other gases or gas mixtures that are less expensive than helium but which would nevertheless limit the appearance of the shielding gas plasma and therefore obtain welding results similar to those obtained with helium, but at a lower cost.
Thus, gas mixtures are commercially available that contain argon and helium, for example the gas mixture containing 30% helium by volume and the rest being argon, sold under the name LASAL(trademark) 2045 by L""Air Liquide(trademark), which make it possible to achieve substantially the same results as helium, for CO2 laser power levels below 5 kW and provided that the power densities generated are not too high, that is to say above about 2000 kW/cm2.
However, the problem that arises with this type of Ar/He mixture is that, for higher laser power densities, it is no longer suitable as the threshold at which the shielding gas plasma is created is then exceeded.
Moreover, it is also paramount for the weld penetration to be at least maintained, or even preferably increased, relative to the same laser welding process using helium.
Furthermore, yet another problem lies in the formation of NOx-type species, harmful to the welder, which must be kept as low as possible.
This is because the metal plasma temperatures, resulting from laser/material interactions as strong as those involved in laser welding, are conducive to the dissociation of nitrogen and oxygen molecules coming from air contamination and lead to the formation of harmful NOx-type species.
Consequently, to avoid or reduce the production of NOx-type species, it is essential to be able to reduce the temperature of the metal plasma resulting from laser welding.
The object of the present invention is therefore to provide a welding gas mixture based on nitrogen and a laser welding process using this gas, able to be used with a laser having a power of up to about 8 kW, which gas leads to the formation of a less hot metal plasma, with a total penetration from 5 to 10% greater than that obtained with the conventional gases used for such power levels, namely typically helium, depending on the power and the nitrogen content of the gas, and to a reduction in the formation of NOx compared with helium by itself.
The solution of the invention is therefore a binary gas mixture for welding using a laser beam consisting of 60% to 80% nitrogen by volume, the remainder (up to 100%) being helium.
Depending on the case, the gas of the invention may include one or more of the following technical features:
it contains less than 80% nitrogen by volume;
it contains less than 70% nitrogen by volume, preferably less than 68% nitrogen;
it less than 67% nitrogen, preferably less than 65% nitrogen.
According to another aspect, the invention also relates to a welding process using a laser beam having a power ranging up to 8 kW, in which a gas mixture according to the invention is used for welding steel, stainless steel or titanium workpieces.
Depending on the case, the process of the invention may include one or more of the following technical features:
the laser is of the CO2 type;
a welding operation is carried out to join two workpieces to be welded together with at least partial penetration, preferably full penetration;
a laser having a power from 0.5 to 7 kW, preferably between 4 and 6 kW, is used;
workpieces having a thickness ranging from 0.4 to 30 mm, preferably from 1 mm to 10 mm, are welded;
the workpieces are made of HYS (High Yield Strength) steel;
the workpieces to be welded have a zinc surface coating, particularly galvanized or electrogalvanized steel plates;
the workpieces to be welded are placed together and lap or butt welded, by backside welding or at an angle, and with or without a bevel;
the welding takes place with a single- or multiple-spot focal spot (impact);
the focal spot is circular or oblong;
the gas flow rate is between 5 l/min and 100 l/min and/or the pressure of the gas is between 1 and 5 bar; and
the nozzle delivering the gas is a lateral or axial nozzle having a diameter ranging from 3 to 30 mm.